Stainless steel tubing with a maximum titanium to carbon ratio of 6

ABSTRACT

A longitudinally welded stainless steel tubing and a method for producing the same wherein a continuous band of stainless steel of the composition, in percent by weight, carbon 0.08 max., manganese 2 max., phosphorus 0.045 max., sulfur 0.045 max., silicon 1 max., nickel 1.25 max., chromium 10 to 13.5, titanium at least 0.10 with a maximum titanium to carbon ratio of 6, nitrogen 0.05 max., iron balance, is formed into a generally cylindrical shape to produce a longitudinal seam, heating the metal adjacent said seam to a temperature above the transformation temperature of said steel and bringing the heated edges into contact and at a pressure to cause welding thereof. The tubing is characterized by improved flareability, along with good corrosion resistance, ductility, fabricability and weldability.

United States Patent [1 1 Bressanelli [111 3,770,394 Nov. 6, 1973 [75] Inventor: Jerome P. Bressanelli, Pittsburgh,

[73] Assignee: Crucible Inc., Pittsburgh, Pa.

[22] Filed: Sept. 14, 1970 [211 Appl. No.: 71,713

[52] US. Cl. 29/193 [51] Int. Cl B32b 15/02 [58] Field of Search 29/193, 183; 148/34; 75/123 M, 126 D, 126 B [56] References Cited UNITED STATES PATENTS 3,493,366 2/1970 Hopkins et a1. 29/183 3,318,690 5/1967 Floreen et a1. 75/123 M 3,352,769 11/1967 Ruben 29/183 3,462,820 8/1969 Maxwell et a1. 29/183 2,597,173 5/1952 Patterson 75/126 D 3,607,246 9/1971 Kalita 75/126 D 3,044,872 7/1962 Hayes et a1. 75/126 D 1/1970 Holtzman 148/34 5/1956 Hobrock 29/193 Primary ExaminerA. B. Curtis Assistant Examiner-C. F. Lefevour Attorney-Clair X. Mullen, Jr.

[57] ABSTRACT A longitudinally welded stainless steel tubing and a method for producing the same wherein a continuous band of stainless steel of the composition, in percent by weight, carbon 0.08 max., manganese 2 max., phosphorus 0.045 max., sulfur 0.045 max., silicon 1 max., nickel 1.25 max., chromium 10 to 13.5, titanium at least 0.10 with a maximum titanium to carbon ratio of 6, nitrogen 0.05 max., iron balance, is formed into a generally cylindrical shape to produce a longitudinal seam, heating the metal adjacent said seam to a temperature above the transformation temperature of said steel and bringing the heated edges into contact and at a pressure to cause welding thereof. The tubing is characterized by improved flareability, along with good corrosion resistance, ductility, fabricability and weldability.

3 Claims, 15 Drawing Figures PATENTEDNUV 6 ms snm 10F T409-HEAT 2 T409 -HEAT F/GI IB STEEL E STEEL 6 STEEL E INVENTO/P STEEL C JEROME I? BRESSA/VELL/ Attorney PATENTEUNIN 6:975 3,770,394 SHEEIEHFG STEEL E STEEL F, STEEL F F/GI 444 E FIG, 4B

INVENTORE Afforney PATENTEDNUY 8 I975 SHEET L [3F 6 STEEL 6 FIG.

//V VE IV T 0/? JEROME F. BRE SSA IVE L .PATENTEUNUV sign 3.770.394

SHEET 5 OF 6 I F/GI 7A STEEL E FIG 7B INVENTOI? JEROME R BRESSA/VELL/ Atinmeu V STAINLESS STEEL TUBING WITH A MAXIMUM TITANIUM T CARBON RATIO OF 6 Stainless steel tubing produced by longitudinal fusion welding finds use in many industrial applications, particularly where corrosive fluids are transferred. In many such applications the fluid passing through the tubing is at high pressure, and therefore it is critical that a strong defect-free weld be provided.

Typically tubing of the character is produced by making a continuous band of stainless steel, for example AISI Type 409, subjecting it to a roll-forming operation wherein the edges thereof are roll-formed to cylindrical shape to provide a longitudinal seam. The adjacent edges defining the seam are then welded together to form the tubing. conventionally, this may be achieved by either induction, resistance or fusion welding, such as tungsten inert gas welding. In either case, however, the edges are heated to a temperature above the transformation temperature of the steel and generally above the solidus temperature thereof. The edges so heated are then brought into contact under pressure sufficient to cause fusion welding upon cooling to ambient temperature. In stainless steel of the type used in these applications the transformation temperature thereof will generally be about 1,400l ,500F if an alloying addition of nickel is present and about l,700-1,800F or higher in the absence of nickel. The melting point of such steels is generally about 2,600 or 2,700F. I 1

When using tubing of this type for applications such as water or waste systems, air conditioning systems and the like it is often required to expand or flare the end of the tubing to increase the diameter thereof to a significant extent for the purpose of producing connecting joints between tubing lengths or to connect a tubing end to various components of the installation in which it is being used. Conventional tubing, which typically is produced from Type 409 stainless steel, tends to crack at the weld zone during flaring and therefore use of tubing of this type has been restricted in applications where flared joints are required to a significant extent.

It is accordingly the primary object of the present invention to provide longitudinally welded stainless steel tubing and a method for producing the same wherein said tubing is characterized by improved flareability in combination with good corrosion resistance, ductility, fabricability and weldability.

This and other objects of the invention, as well as a complete understanding thereof, may be obtained from the following description, specific examples and drawings, in which:

FIG. 1 is photographs showing conventional flared tubing and tubing produced in accordance with the present invention;

FIG. 2 is photographs comparing high frequency resistance welded tubing of the invention with conventional tubing when expanded with a bell-shaped mandrel;

FIG. 3 is a photograph of high frequency resistance welded tubing produced in accordance with the invention with the end thereof formed to produce a flare and bent back on itself;

FIG. 4 is photographs of tungsten-inert-gas-welded tubing produced in accordance with the invention and subjected to drastic end flaring;

FIG. 5 is a photomicrograph of conventional Type 409 stainless steel tubing produced by high frequency resistance welding showing both the weld zone microstructure and the microstructure of the remainder of the tubing at a magnification of X;

FIG. 6 is a photograph showing a typical microstructure in the weld zone of A181 Type 409 stainless steel tubing produced by high frequency welding and showing the large ferrite grain size and eutectic-like network adjacent the titanium carbonitride particles at a magnification of 1,000X;

FIG. 7 is photomicrographs showing the microstructure of the weld zone of high frequency resistance welded tubing produced in accordance with the present invention, at a magnification of l,000X;

FIG. 8 is a photomicrograph of high frequency resistance welded tubing produced in accordance with the invention both of the weld, heat affected zone and remainder of the stainless steel tubing, at a magnification of 50X-; and

FIG. 9 is a photomicrograph of high frequency resistance welded tubing produced in accordance with the invention and showing the microstructure of the weld, heat afiected zone and remainder of the tubing at a magnification of 50X.

It is found that improved flareability, in accordance with the present invention, may be obtained, independent of the welding practice used, by critically restricting the titanium content of the stainless steel and the ratio of titanium to carbon, along with balancing the composition otherwise with respect to ferrite and austenite promoting elements so that upon welding a tough, predominantly martensitic weld is produced with the remainder of the tubing being substantially fully ferritic. This result of improved flareability-specifically the fact that upon flaring the tubing will not crack at the weldwhich is achieved by providing a predominantly martensitic weld is totally unexpected in that it is well known that martensite is less ductile than ferrite and consequently where forming to an extensive degree is desired one would not be led to believe that the presence of martensite would enhance the forming properties. It is believed that the beneficial effect of providing a predominantly martensitic weld with respect to flareability of the tubing is brought about, at least in part, by the higher strength of the weld in relation to the remainder of the tubing, which is substantially fully ferritic. A martensitic weld, with the remainder of the tubing being ferritic, alters the strain distribution about the tubing circumference during flaring. Therefore, the predominantly martensitic weld is much stronger than the remainder of the tubing which is ferritic so that most of the strain introduced during the expanding of the tubing incident to flaring occurs preferentially in the weaker ferritic metal to reduce the cracking tendency in the weld.

Broadly in the practice of the invention stainless steel in the form of a continuous band or strip of the following composition, in weight percent, is used:

Broad Preferred Element Weight, Element Weight, Carbon 0.08 max. Carbon 0.02 to 0.07 Manganese 2 max. Manganese 0.20 to 1.25 Phosphorus 0.045 max. Silicon 0.50 max. Sulfur 0.045 max. Nickel 0.10 to 1 Silicon 1 max. Chromium 11 to 12.5 Nickel 1.25 max. Aluminum 0.5 max. Chromium 10 to 13.5 Titanium At least 0.10

with a max. Aluminum 0.5 max. titanium to carbon I ratio of 5 Titanium At least 0.10 with Iron Balance a max.

titanium to carbon ratio of 6 Nitrogen 0.05 max. lron Balance The band of the above stainless steel composition is, by conventional roll-forming techniques, formed to a generally cylindrical shape so that the adjacent edges thereof form a longitudinal seam. The edges are heated to a temperature at least above the transformation temperature of the steel and generally to a temperature above the solidus temperature of the steel. Typically the transformation temperature will be about 1,400" to 1,500F if nickel is present as an alloying addition and 1 ,700 to 1 ,800F or higher without nickel. The melting point of the steel will be about 2,600 to 2,700F. Upon heating to temperature the edges are brought into contact under pressure sufficient to cause fusion thereof so that upon cooling to ambient temperature an integral longitudinal weld is achieved. The resulting tubing will be characterized by a longitudinal weld zone of predominantly martensite with the remainder of the tubing being substantially fully ferritic. In addition, the grain size in the weld will be much finer than that conventionally produced; specifically the maximum grain size will be about ASTM 6 or finer. In view of the low titanium to carbon ratio and preferably also the higher than normal nickel content of the composition used in the invention, the weld structure will have substantially fewer titanium carbonitride particles and will have a substantial absence of the eutectic-like network about the titanium carbonitride particles that are present. It is believed that the absence of a eutectic network improves the flareability in that typically such a network serves as a site of crack initiation during forming.

Carbon contents above 0.08 max. are undesirable in that such decreases the toughness of the martensite in the weld, unless unduly large amounts of titanium are used. Furthermore, increasing carbon content above 0.08 max. and simultaneously increasing titanium content, while still maintaining a titanium to carbon ratio within our broad or preferred range, is undesirable, because it introduces unnecessarily large quantities of titanium carbonitride particles in the weld zone which, in themselves, can be detrimental to flareability.

Although our preferred compositional range includes a 0.02 percent minimum limit for carbon, still lower carbon contents than this would be desirable from a flareability standpoint, but would require the use of special melting techniques which would adversely affect the production cost of the steels.

In our broad range of composition, the titanium to carbon content is restricted to below 6 to 1. if the nickel content of the steel is kept low (0.5 percent or less), the maximum limit on the titanium to carbon ratio must be further restricted, as an example, to a maximum ratio of about 5 to l in order to insure good flareability. If the nickel content of the steel is greater than 0.5 percent, but less than about 1.25 percent, titanium to carbon ratios up to the 6 to 1 maximum level of our recommended broad compositional range can be used.

dominantly martensitic riature of the weld metal is maintained.

A certain minimum amount of chromium (at least 10 percent) is required to insure good corrosion resis tance. Although increasing amounts of chromium continually increase corrosion resistance, chromium contents above about 13.5 percent are undesirable. C h r o v mium promotes ferrite in the weld; to compensate for the effects on weld structure of chromium contents above about 13.5 percent, unduly large amounts of nickel and/or manganese would be required to maintain the desired predominantly martensitic structure of the weld.

The level of nickel and manganese contents in the steel is dictated primarily by the need to maintain a predominantly martensitic weld structure. In general, as the combined level of the ferrite forming elements (chromium, silicon, titanium, aluminum, etc.) increases, increasing amounts of nickel and/or manganese is required to maintain the structural balance of the weld. However, excessive amounts of nickel and manganese beyond those of the above-listed composition ran es are undesirable, because low annealed hardness (less thamoiitlii ll) is a requirement for strip from which tubing is made. Excessively high nickel and manganese contents retard softening during annealing to the extent that the desired low hardness cannot be obtained practically under commercial annealing conditions. In addition, large amounts of these elements, particularly nickel, signaificantly increase the cost of the steel.

Silicon and aluminum are very useful elements in steelmaking for deoxidation purposes and for improving oxidation resistance, but with our invention excessive amounts of silicon and aluminum beyond about 1.0 percent and 0.5 percent, respectively, significantly lower weld toughness and ductility. Furthermore, excessively high silicon and aluminum add to the cost of the alloy by requiring the use of higher than desirable amounts of nickel and/or manganese to maintain the desired predominantly martensitic structural balance of the weld metal.

With respect to flareability in Table 1 two typical Type 409 compositions were compared with Steels C and E in the form of 2 in. diameter tubing having a 0.048 in. wall thickness. The results reported are the percent of expansion in diameter without failure by the use of a conical shaped mandrel having a 60 taper. All of the tubing was produced by conventional rollforming and resistance welding. Steels C and E represent tubing produced in accordance with my invention. It should be noted that the maximum expansion of the tubing produced in accordance with the invention ranged from 50 to 60 percent; whereas, the flareability of the conventionally produced tubing attained a maximum of only 20 percent.

TABLE I Composition (weight percent) F1are- Ti/C ability Grade C Mn Si Ni Cr Ti N ratio (percent) T409 heat 1 0.050 0.38 0.40 0.27 11.95 0.47 0.02 9.4 -20 T409 heat .050 .62 .41 .28 11.50 .60 .01 8.6 15-20 Steel (low Ti/(. .065 .45 .27 .25 11.85 .21 .01 3.2 50 Steel E (low Tl/(I, high N .060 .62 .26 .67 12.15 .26 .02 4.3 55-60 FIGS. 1A and 1B are photographs of the tubing constituting Heats l and 2, respectively, as reported in Table I, after flaring. FIGS. 1C and 1D represent Steels C and E, as reported in Table I after flaring. It is obvious from these photographs that even though the conventional tubing was subjected to less severe expansion than the tubing produced in accordance with the invention, nevertheless its failure by weld cracking was much more pronounced than that of the tubing of the invention even though subjected to greater expansion.

Table [1 lists compositions 1 through 7 which were conventionally formed into 2 in. diameter tubing by high frequency resistance welding, and A through F of Table II represent high frequency resistance welded tubing produced in accordance with the present invention. All of the tubing was subjected to a bell-shaped flaring operation and rated. A good rating indicates that the tubing achieved the bell-shaped flare without weld cracking, and poor means that cracking occurred during forming. During the forming operation each tube was subjected to a maximum expansion in diameter of 25 percent.

FIG. 2A shows conventional tubing 1 after flaring. It may be seen that drastic cracking along the weld resulted from this expansion. In contrast FIG. 2B shows tubing E, which was produced in accordance with the present invention. It may be seen that the weld of this tubing remained intact upon being subjected to the identical expanding operation as imparted to the conventionally produced tubing shown in FIG. 2A.

The result of the invention is achieved regardless of the specific welding technique employed. For example, FIG. 3 shows tubing produced'by resistance welding of I the composition identified as E in Table I successfully subjected to an extremely drastic flaring operation wherein the end of the tubing was bent back on itself without cracking.

flaring application, as shown in the photograph identified as FIG. 4A, in addition to a more drastic flaring operation, as shown in the photograph identified as FIG. 4B. In both cases the weld remained intact.

As may be seen from FIG. 5 conventionally produced tubing from Type 409 stainless steel shows a relatively coarse grain ferritic structure. In addition, as shown in FIG. 6, the structure is characterized by titanium carbonitride particles in a eutectic-like network. The eutectic network is believed to result from the rapid and, in most cases, incomplete dissolution of the titanium carbonitride particles at the high temperatures prevailing during the welding operation. With the conventionally produced tubing it is believed that the eutectic-like network serves as a site for crack initiation during flaring of the tubing. Upon initiation of a crack it rapidly propagates through the relatively coarse-grained ferrite of the weld zone.

In contrast, with the practice of the present invention the low titanium to carbon ratio, preferably combined with nickel in an amount above 0.5 percent nickel results in substantially fewer titanium carbonitride particles and the absence of the eutectic-like network. This is demonstrated by the photomicrographs of FIGS. 7A and 7B which are the weld zones of tubing identified as C and E produced in accordance with the present invention. Also, with the practice of the invention the weld of the tubing is predominantly martensitic and the grain size is much finer than that conventionally obtained. For this purpose the photomicrographs of FIGS. 8 and 9, which are of tubing produced in accordance with the present invention, should be compared with that of FIGS. 5 and 6, which are photomicrographs of tubing produced conventionally. FIG. 8 is a photomicrograph of tubing identified as C and produced in accordance with the present invention and shows the microstructure of the weld, heat affected zone and base TABLE 11 Composition (weight percent) 7 Ti/C Flare- Grade H6 C Mn Si Ni Cr Ti ratio ability T409 1 0.050 0.38 0.40 0.27 11.95 0.47 9.4 Poor. T409.... 2... .050 .62 .41 .28 11.50 .60 8.6 Do. T409.... 3... .061 .53 .21 .28 11.35 .50 8.2 Do. T409.... 4... .062 .53 .21 .28 11.03 .50 8.1 Do. T409.... 6... .065 .53 .21 .28 10.98 .50 7.7 Do. T409... 6... .074 .54 .44 .27 11.49 .49 6.6 Do. T409 7... .080 .45 .59 .23 all-2. .44 5.5 Do. .075 .46 .23 .24 10.96 .31 4.1 Good. .071 .48 .24 .24 11.35 .28 3.9 Do. .065 .42 .27 .25 11.85 .21 3.2 Do. .077 .47 .62 .24 1 1.08 .23 3.0 Do. .060 .62 .26 .67 12.15 .26 4.3 Do. .070 .32 .70 1 1.90 .21 3.0 Do.

FIGS. 4A and 4B show tubing identified as F in Table II produced by tungsten-inert-gas welding, which tubing likewise was successfully subjected to both a typical metal. At the same magnification as that of FIG. 5 the finer grain structure, particularly of the weld, is quite evident. FIG. 9 shows the microstructure of tubing at Element Percent Carbon 0.08 max. Manganese 2 max. Phosphorus 0.045 max. Sulfur 0.045 max. Silicon 1 max.

Nickel 1.25 max. Chromiun 10 to 13.5 Aluminum 0.5 max. Titanium At least 0.l0 with a max. titanium to carbon ratio of 6 Nitrogen 0.05 max.

Iron Balance said tubing having a longitudinal weld zone of predominantly martensite with the remainder of said tubing being substantially fully ferritic.

2. The tubing of claim 1 wherein said predominantly martensitic longitudinal weld zone has a maximum grain size of about ASTM 6 or finer.

3. The tubing of claim 1 wherein said stainless steel composition is, in percent by weight:

Element Percent Carbon 0.02 to 0.07

Manganese 0.20 to 1.25

Silicon 0.50 max.

Nickel 010 to l Chromium ll to 12.5

Aluminum 0.5 max.

Titanium At least 0.l0 with a max. titanium to carbon ratio of 5 Iron Balance 

2. The tubing of claim 1 wherein said predominantly martensitic longitudinal weld zone has a maximum grain size of about ASTM 6 or finer.
 3. The tubing of claim 1 wherein said stainless steel composition is, in percent by weight: Element Percent Carbon 0.02 to 0.07 Manganese 0.20 to 1.25 Silicon 0.50 max. Nickel 0.10 to 1 Chromium 11 to 12.5 Aluminum 0.5 max. Titanium At least 0.10 with a max. titanium to carbon ratio of 5 Iron Balance 